# std::vector<T,Allocator>::operator[]

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< cpp‎ | container‎ | vector

reference operator[]( size_type pos ); | (1) | (constexpr since C++20)
const_reference operator[]( size_type pos ) const; | (2) | (constexpr since C++20)

Returns a reference to the element at specified location pos. No bounds checking is performed.

### Parameters

pos | - | position of the element to return

### Return value

Reference to the requested element.

### Complexity

Constant.

### Notes

Unlike std::map::operator[], this operator never inserts a new element into the container. Accessing a nonexistent element through this operator is undefined behavior.

### Example

The following code uses operator[] to read from and write to a std::vector<int>:

Run this code

    #include <vector>
    #include <iostream>

    int main()
    {
        std::vector<int> numbers{2, 4, 6, 8};

        std::cout << "Second element: " << numbers[1] << '\n';

        numbers[0] = 5;

        std::cout << "All numbers:";
        for (auto i : numbers)
            std::cout << ' ' << i;
        std::cout << '\n';
    }

    // Since C++20 std::vector can be used in constexpr context:
    #if defined(__cpp_lib_constexpr_vector) and defined(__cpp_consteval)
    // Gets the sum of all primes in [0, N) using sieve of Eratosthenes
    consteval auto sum_of_all_primes_up_to(unsigned N)
    {
        if (N < 2)
            return 0ULL;

        std::vector<bool> is_prime(N, true);
        is_prime[0] = is_prime[1] = false;

        auto propagate_non_primality = [&](decltype(N) n)
        {
            for (decltype(N) m = n + n; m < is_prime.size(); m += n)
                is_prime[m] = false;
        };

        auto sum{0ULL};
        for (decltype(N) n{2}; n != N; ++n)
            if (is_prime[n])
            {
                sum += n;
                propagate_non_primality(n);
            }

        return sum;
    } //< vector's memory is released here

    static_assert(sum_of_all_primes_up_to(42) == 0xEE);
    static_assert(sum_of_all_primes_up_to(100) == 0x424);
    static_assert(sum_of_all_primes_up_to(1001) == 76127);
    #endif

Output:

    Second element: 4
    All numbers: 5 4 6 8

### See also

at | access specified element with bounds checking (public member function)

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